(Portions of the technical material contained in this section may not be prior art.)
A common element in packaging electronic IC devices is a plastic housing. In the most typical IC plastic package, electronic components are assembled on a metal leadframe and a polymer is molded over the assembly to encapsulate the device and portions of the leadframe. The leadframe serves not only to support the electronic components, but has metal tabs that extend from the overmolded plastic and provide a means to electrically connect to the encapsulated electronic components.
Recent modifications of the molded plastic IC package include an air cavity design wherein the housing for the package is plastic, but is pre-molded over a lead frame before the IC device is assembled into the package. This design offers the advantage that the IC chip is not exposed to the rigors and heat of the overmolding step used to produce the plastic encapsulated (overmolded) package. In this design the IC device environment may be an air cavity, or the cavity may be filled after the IC chip is die and wire bonded. The cavity filling may be any polymer, including polymers that cure at low temperatures. The choice of filling material is wider than the choices available in the case of overmolded plastic packages, since the choice is independent of the material used for the pre-molded plastic housing. For example, the material is often chosen for low loss at RF frequencies so that less power is dissipated in the packaging material. Thus in a pre-molded plastic cavity package, the IC chip may still be polymer encapsulated, but the heat required for a typical overmolding step may be avoided.
Leadframes of the prior art typically are square or rectangular and have a center paddle to which the semiconductor chip is die bonded. Leadframes are typically stamped from copper or copper alloy sheets. (In the following discussion, use of the term copper is intended by inference to include copper alloys). The leads that provide electrical interconnection extend from the sides of the paddle, often along two opposing edges of the leadframe. The number of leads may vary widely. Common RF power devices, for example, RFLDMOS devices, may have only a few leads, one per side for each transistor. Traditionally the power RF transistor packages have been fabricated from ceramic and powder metal composites that provide the strength needed for the bolt down flange. These packages are expensive.
Whereas the use of copper as the leadframe material offers the advantage of high electrical and thermal conductivity, there are device applications where a copper leadframe is not suitable. For example, in some IC device package designs, the leadframe has a large paddle, to which the IC die is attached, and further comprises tabs extending from the sides of the paddle. The tabs on two opposing sides serve as electrical connectors. The tabs on the other two opposing sides serve as hold-down members that allow the IC package to be attached to a support structure. This type of package is commonly used for RF power devices, such as LDMOS transistors, that have only one, or a few, leads per side. If copper is used as the leadframe material for this type of package, the copper has to be made much thicker in order to match the strength of the leadframe in the ceramic/metal package that the plastic package replaces.
Thus both the tab leads and the hold-down members may require a material with more strength than standard copper leadframes provide. Accordingly, the leadframe material in these devices should be a stronger metal. Steel is favored from a cost standpoint. It is recognized that the comparatively low electrical conductivity of steel is of limited consequence in a leadframe where the paddle is not a conductor member. However, the lower thermal conductivity of steel compared with copper is significant in a power transistor device. A major issue in the design and performance of power transistors is heat dissipation. The steel paddle may not afford an effective heat sink for some power transistors.